You'll still want to level up all 3 trees, by the time you hit 50 as long as you're grabbing skyshards and doing public dungeons you'll have more than enough skill points to fill out all 3 class trees, your armor tree, your weapon tree(s), and then some.

If you want to play a nightblade but dont want to focus on stealth, or dont want the typical dual wield assassin image, I'd suggest using a 2 handed weapon. I use a greatsword + bow on my nightblade, have been for some time, and I love it. Nightblade gives a lot of passive bonuses to damage resistance, stamina regeneration, critical chance, and critical damage, all of which benefit a 2handed playstyle.

Even if you're not looking at playing stealthy, you should still level up shadow cloak. One of the morphs for shadow cloak is shadowy disguise, which causes your next attack to have a 100% critical chance. That, combined with a buffed wrecking blow using a 2handed weapon, puts out a massive amount of damage, enough to one-shot kill most enemies in pve.

You'll want to pick up veiled strike (morphed into surprise attack), teleport strike (morphed into ambush), and hidden blade (morphed into killer's blade). These three nightblade abilities all use stamina and weapon damage when morphed to those variants, and can benefit a stamina build greatly. Ambush is a great closer, dealing more damage than critical charge and causing your next attack to do 20% more damage. Surprise attack stuns your target for 4 seconds and reduces their armor by a large amount when you use it from stealth, and it's a good instant stamina-based attack for when you dont have time to wind up wrecking blow. Killer's blade is a finishing/execution style attack for use on low health targets, and will restore some health to you when you kill someone with it.

You get your second weapon slot at level 15, so you'll be able to use two different weapon types at that level and quickly switch between them.

Hey,

If you stay on dual wield your 2H will not level meaning your abilities will be weak,

I always wear 5pces of the armour that really helps my build and then 1 and 1, eg NB Stam bld, 5med, 1 light and 1 heavy.

During levelling, mostly about any build can do ( unless of cos if you are just slapping abilities with abandon) but it is good practice to know what your endgame build is gonna be like. As of now, dual wield has amazing aoe stamina abilities but it falters in terms of sustain and utility compared to 2handers.

However, NBs have really good support abilities for stamina builds. Like you said, siphon really allows you to have good sustain and help in increasing damage output. Experiment with the other class lines as well, shadow has really amazing mitigation Tools and assassin has really good synergy with weapon skill lines.

Armor wise.. Having a full set medium armor will definitely help with dps. You can also complement that set with either two pieces of heavy or perhaps with one light and one heavy. You can go heavy armor, like what I did with my Reaver build. Its pure melee though and its really for levelling, rather than for endgame but I like how it turns out.

Not really tree advice but fellow noob advice......if you see a bunch of higher levels fighting whats on my icon, thenusing a bow to get some hits in makes for good stuff, if you see something similar later and you go in all Double Blade gungho....like me your probably going to die.